[PATCH 6/6] fdtdec: apply DT overlays from bloblist

[Michal Simek]

On 6/18/25 16:59, Raymond Mao wrote:
> During FDT setup, apply all existing DT overlays from the bloblist
> to the base FDT if bloblist is being used for handoff from previous
> boot stage.

More technical details would be good to have here.
Especially that the way what you are using is to expand DT

> 
> Signed-off-by: Raymond Mao <raymond.mao at linaro.org>
> ---
>   lib/fdtdec.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 56 insertions(+)
> 
> diff --git a/lib/fdtdec.c b/lib/fdtdec.c
> index c38738b48c7..3a218d8c94a 100644
> --- a/lib/fdtdec.c
> +++ b/lib/fdtdec.c
> @@ -1687,6 +1687,60 @@ void fdtdec_setup_embed(void)
>   	gd->fdt_src = FDTSRC_EMBED;
>   }
>   
> +static int fdtdec_apply_dto_from_blob(void **blob)
> +{
> +	int ret;
> +	struct fdt_header *live_fdt;
> +	size_t new_fdt_size, blob_size;
> +	int expand_by;
> +
> +	if (!CONFIG_IS_ENABLED(OF_LIBFDT_OVERLAY) ||
> +	    !CONFIG_IS_ENABLED(BLOBLIST))
> +		return -EPERM;
> +
> +	live_fdt = bloblist_find(BLOBLISTT_CONTROL_FDT, 0);
> +	ret = fdt_check_header(live_fdt);
> +	if (ret)
> +		return ret;


Why is this needed? You already have gd->fdt_blob which has been checked.


> +
> +	ret = fdt_check_header(*blob);
> +	if (ret)
> +		return ret;
> +
> +	blob_size = fdt_totalsize(*blob);
> +
> +	/* Expand the FDT for spare spaces to apply the overlay */
> +	new_fdt_size = fdt_totalsize(live_fdt) + blob_size + 0x400;

Some notes about this 0x400 would be good. Do you see issue if you don't use 
this value? Is file DTB after expansion actually bigger then origin+overlay that 
you need to add 0x400 magic value here?


> +	ret = bloblist_resize(BLOBLISTT_CONTROL_FDT, new_fdt_size, &expand_by);
> +	if (ret)
> +		return ret;

You do resize inside tl list.. That should be described in commit message 
because my exception was that this location can be read only from NS world.

I didn't run this on HW and likely I should do it to understand this better
but isn't this moving CONTROL_FDT entry to different location which will have 
bigger space for applying overlay?

Isn't it also worth to check if existing CONTROL_FDT has enough space inside 
that new overlay can fit there before resizing it?


> +
> +	/* The blob is shifted if it was following the FDT */
> +	if (*blob > (void *)live_fdt)
> +		*blob += expand_by;
> +
> +	ret = fdt_open_into(live_fdt, live_fdt, new_fdt_size);
> +	if (ret)
> +		return ret;
> +
> +	ret = fdt_overlay_apply_verbose(live_fdt, *blob);
> +	if (ret)
> +		return ret;
> +
> +	/* Shrink the excessive spaces */
> +	fdt_pack(live_fdt);
> +	new_fdt_size = fdt_totalsize(live_fdt);
> +	ret = bloblist_resize(BLOBLISTT_CONTROL_FDT, new_fdt_size, &expand_by);
> +	if (ret)
> +		return ret;

I didn't try to run on HW but don't you need to also adjust gd->fdt_blob to 
point to current location where fdt is?

Thanks,
Michal